planner-agent: use env_file, keep only ANTHROPIC_API_KEY in environment

All runtime vars (REDIS_URL, OLLAMA_HOST, OLLAMA_MODEL, NODE_NAME,
COOLDOWN_SECONDS, RUNTIME_PATH) are sourced from the host-local
/opt/homelab/config/planner-agent/.env via env_file.
Only ANTHROPIC_API_KEY stays in environment (not in env_file — secret
injected at runtime by the operator when needed).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/planner-agent/docker-compose.yml

@@ -3,16 +3,14 @@ services:
     build: .
     container_name: planner-agent
     restart: unless-stopped
+    extra_hosts:
+      - "host-gateway:host-gateway"
     volumes:
       - /opt/homelab:/opt/homelab
+    env_file:
+      - /opt/homelab/config/planner-agent/.env
     environment:
-      - REDIS_URL=${REDIS_URL:-redis://100.108.208.3:6379}
-      - OLLAMA_HOST=${OLLAMA_HOST:-http://100.108.208.3:11434}
-      - OLLAMA_MODEL=${OLLAMA_MODEL:-qwen2.5:7b}
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY}
-      - NODE_NAME=${NODE_NAME:-solaria}
-      - COOLDOWN_SECONDS=${COOLDOWN_SECONDS:-300}
-      - RUNTIME_PATH=${RUNTIME_PATH:-/opt/homelab}
     healthcheck:
       test: ["CMD", "/bin/sh", "/app/healthcheck.sh"]
       interval: 60s